Cr.sub.2 O.sub.3 is a component of a large number of pigments of various crystal structures. The colors of these pigments extend over a wide range of visible light. Color shades from ochre to dark brown are obtained in the spinel system (ZnMg) (Fe, Cr).sub.2 O.sub.4 by varying the cations. Color shades from light green to dark blue may be obtained in the system (ZnCo) (Cr, Al).sub.2 O.sub.4. Black pigments are also obtained in the spinel system. The color is produced by varying compositions of the oxides of Cu, Cr, Fe, Mn, Co and Ni. The hematite system (Fe, Cr).sub.2 O.sub.3 gives rise to various black-brown pigments, depending on the composition.
These pigments are technically important for use in lacquers, plastics, enamels and ceramics.
The afterannealing of pure Cr.sub.2 O.sub.3 or mixtures of Cr.sub.2 O.sub.3 and from 0.1 to 5% of TiO.sub.2 gives rise to green pigments crystallizing in the corundum lattice which are used for special purposes, e.g. in the field of plastics, lacquers and refractory materials, in which primary particles of increased size are required.
All these Cr.sub.2 O.sub.3 -containing pigments are produced by wet or dry mixing the corresponding starting oxides, hydroxides or compounds which give rise to oxides under the reaction conditions, annealing the mixtures in the oven and then grinding them down to the required particle size (DE-C 2 338 012 and DE-C 2 320 806).
These methods, however, have the disadvantage that annealing produces furnace clinker containing substantial quantities of Cr(VI). Grinding these pigments must therefore be followed by washing and the wash water, which contains Cr(VI), must be subjected to an expensive reductive after-treatment wherein the Cr.sup.3+ must be precipitated as Cr(OH).sub.3 and removed.
In addition, subsequent drying of the washed pigments or coloring substances is likely to be accompanied by partial back formation of chromate.
Coloring substances free from chromium(VI) and containing Cr.sub.2 O.sub.3 may be obtained according to DE-A 3 118 966 by the addition of antimony(III) oxide during annealing. Antimony(III) oxide is, however, a physiologically potentially harmful material.
It is therefore an object of the present invention to provide a process which is free from the disadvantages described above.